1. Field of the Invention
The present invention relates to the separation of compounds having olefinic unsaturation from other compounds by means of complexing the olefinically unsaturated compounds with transition metal salts, particularly silver salts, in aqueous solutions and liberating the olefinically unsaturated compounds by decomposing the complex. More particularly, the invention provides a means to prevent the formation of dangerous unstable metal acetylides.
2. Related Art
The separation of olefins from corresponding paraffin is difficult by the conventional methods of the prior art, e.g., lean oil extraction and/or fractional distillation.
It is known that several transition metals in the +1 valence state form moderately stable complexes with compounds having olefinic unsaturation. Among the transition metals which exhibit the property are silver, copper (+1), mercury (+1), rhodium (+2), palladium (+2) and platinum (+2). All of these metals with the exception of silver exhibit valences of +2 and may form very stable complexes in the +2 valence. Copper will form more stable complexes in the +2 valence state.
The commercial significance of separating compounds having olefinic unsaturation from other compounds using the complexing property of the transition metals depends on the ability to easily decompose the complexes and release the olefinic compound. The metals other than silver and copper are more expensive and their use in large quantities associated with this type of separation present prohibitive capital costs.
Silver, on the other hand, is of moderate cost and has only one valence state. The silver complexes are soluble in water whereas the remainder of the organic steam, mainly hydrocarbons, has only slight solubility in water. This is described in U.S. Pat. No. 4,328,382 issued May. 2, 1982. U.S. Pat. No. 4,174,353 issued Nov. 13, 1979, discloses the removal of silver acetylide in the aqueous silver nitrate solution with silver permanganate or ozone and H.sub.2 O.sub.2 to control Ag precipitation.
Presently there is no commercial application of this technology for olefin separation.
"Selective Olefin Recovery", Petrochemicals Technology Quarterly, pages 117-122, Spring 1997, Barchas, describes a process for upgrading refinery-grade (about 70%) propylene to polymer-grade (99%+) propylene. In this process gaseous feed enters a packed section and is contacted with descending silver nitrate solution. A complex is formed between the silver and the olefin which is then dissolved in the aqueous solution to form a "rich" solution. The complex containing solution is then flashed by pressure letdown into a drum which releases the olefin. The article also describes a process using a gas permeable membrane, through which the olefin containing gas passes into a silver nitrate solution to form a silver complex in solution while the paraffin gases pass out unaffected.
Barchas particularly points out two substantial problems with his silver nitrate/olefin recovery process. The first is the presence of hydrogen in the olefin containing feed gas. Hydrogen reduces the silver salt and results in the precipitation of silver metal in the system. Barchas advises silver nitrate cannot be used unless the hydrogen is removed or silver somehow prevented from precipitation. However, no solution is offered.
The second problem is the formation of compounds of silver with some feed contaminants, namely diolefins, acetylenes, sulfides and chlorides. The solid sulfides and chlorides are an inconvenience and cause loss of available silver and must be removed by filtration. However, the article, although noting the hazard of solid silver acetylides, may be considered to understate the problem.
Silver acetylides are extremely unstable, particularly if they become dry. Thus, this solid material passing through the system, even if filtered out, may explode within the olefin plant. This is an extreme hazard, and in this format is not likely to result in any commercial use of the silver salts for olefin removal or recovery.
If these two primary problems can be eliminated, then the use of silver salts for selective olefin separation from saturated compounds would substantially reduce the cost of capitalization and operation from the prior commercial systems.
It is an advantage of the present invention that moderately stable silver complexes are formed with hydrocarbon mixtures including compounds having olefinic unsaturation. It is a particular advantage that compounds having olefinic unsaturation are easily separated from saturated compound having the same number of carbon atoms. It is a further advantage of the present process that silver is not reduced and precipitated by hydrogen present in the organic feed to the process. It is a particular advantage of the present process that unstable compounds of silver and acetylenes are not formed during the olefin separation process.